Place the summer squash slices in a steamer basket and steam for 3-4 minutes or until tender. Transfer the slices to a plate and set aside. Place the zucchini slices in the steamer basket and steam for 2-3 minutes or until tender. Lightly oil a large casserole dish. Alternately layer the summer squash and zucchini in the casserole dish. Sprinkle with 1/2 t. garlic powder and season with salt and pepper, to taste. In a small bowl, place the remaining garlic powder and remaining ingredients, and stir well to combine. Sprinkle the mixture over the vegetables and bake at 350 degrees for 15-20 minutes or until lightly browned on top.